(1) Field of the Invention
The present invention relates to a method for controlling fungal diseases in grasses using a unique strain of Pseudomonas aureofaciens. In particular, the present invention relates to novel strains of Pseudomonas aureofaciens which inhibit the fungal disease Dollar Spot carried by the fungus (Sclerotinia homoeocarpa) which is a major pathogen of turfgrass.
(2) Description of Related Art
Techniques currently employed for the management of fungal turfgrass diseases are highly dependent on the application of chemical fungicides. A broad array of chemical fungicides are currently available for the management of these fungal pathogens. However, the development of resistance to certain classes of chemical fungicides and the failure of agrochemical companies to re-register currently used chemical fungicides with the EPA due to cost and environmental concerns, have promoted research into identification of new chemical fungicides as well as research into alternative disease management strategies. The two major alternatives to the use of chemical fungicides being examined are biological controls and organic or composted fertilizers.
Development of new chemical fungicides generally occurs through the mass screening of novel synthetic compounds. Utilization of antifungal compounds produced by microbial organisms, such as antibiotics, have been highly exploited in the development of medicinal compounds. Application of medicinal antibiotics for the management of plant diseases has been restricted due to concerns of the development of resistance to these compounds by potential human pathogens.
Several bacteria have been identified as producing a variety of classes of compounds that are antifungal in nature, including enzymes, siderophores, hydrogen cyanide, ethylene, and antibiotics. Although all of these compounds have been implicated in biological control activity by bacteria, the commercial application of enzymes for plant disease management is not likely due to their sensitivity to environmental conditions. Another class of compounds that would not be feasible for study are volatile compounds such as hydrogen cyanide and ethylene.
The use of bacteria as biological control agents is one of the fastest growing fields of research in disease management. The concept of the management of disease through the application of soilborne bacteria is attractive due to its sensitivity to environmental concerns. However, significant breakthroughs yielding biological controls that provide consistent disease management have not yet been realized.
Pseudomonas aureofaciens is a gram negative rod shaped bacterium, possessing one or more flagella, strictly aerobic, and chemoorganotrophic. P. aureofaciens is included in the class of fluorescent pseudomonads and was included taxonomically as a biovar of Pseudomonas fluorescens by Stanier et al (Journal of General Microbiology 43:159-271 (1966)). Inclusion of P. aureofaciens in the group of fluorescent pseudomonads is based on the ability of most strains to produce fluorescent pigment pyoveridin. The name "aureofaciens" literally means to "make golden" which refers to its ability to turn artificial media to a orange-gold color. This color is caused by production of non-fluorescent phenazine pigments. Phenazine pigments reported to be produced by P. aureofaciens are phenazine-1-carboxylic acid (PCA), phenazines 2-oxo phenazine and 2-oxo phenazine-1-carboxylic acid (Trutko, S. M., et al., Biokhimima 54:1329-1336 (1990)). Evidence has been presented that the role of phenazine compounds produced by P. aureofaciens allows for the removal of excess reducing equivalents from NADH and NADPH under substrate and/or oxygen limitations.
Management of turfgrass diseases has conventionally been conducted through the employment of proper cultural practices and the application of chemical fungicides (Vargas, J. M., Management of turfgrass diseases. Burgess Publishing Co. Minneapolis, Minn. 204 (1981)). One of the more recent instruments of disease management to be examined is the utilization of bacteria and fungi as biological controls. Investigations into the application of biological controls have been conducted toward the management of the turfgrass diseases, brown patch (Rhizoctonia solani) (Burpee, L. L., et al., Phytopathology 74:692-694 (1984)), dollar spot (Sclerotinia homoeocarpa) (Goodman, D. M., and Burpee, L. L., Phytopathology 81:1438-1446 (1991); Haygood, R. A., et al., Phytopathology 80:435 (1990); and Nelson, E. B., et al., Plant Disease 75:510-514 (1991)), fairy rings (many basidiomycetes) (Smith, J. C., Fairy rings: Biology, antagonism and possible new control methods. Pages 81-85 in: Advances in Turfgrass Pathology. P. O. Larson and B. G. Joyner, eds. Harcourt Brace Jovanovich, Duluth, Minn., 197 pp (1980)), necrotic ring spot (Leptosphaeria korrae) (Melvin, B. P., Biological and cultural management of summer patch and necrotic ring spot. Ph.D. Dissertation (1991)), summer patch (Magnaporthe poae) (Melvin, B. P., Biological and cultural management of summer patch and necrotic ring spot. Ph.D. Dissertation (1991); and Thompson, D. C., et al., Phytopathology 82:1123 (1992)), take-all (Gaeumannomyces graminis (Wong, P. T. W., et al., Annals of Applied Biology 92:191-197 (1979)), and typhula blight (Typhula incarnata and Typhula ishikariensis) (Burpee, L. L., et al., Plant Disease 71:97-100 (1987)).
The role of microflora in relation to the reduction of disease severity was brought to light with the identification of "suppressive soils". "Suppressive soils" refers to soils which reduce the level of disease intensity to a particular pathogen (Rovira, A. D., et al., The nature and mechanism of suppression. Pages 385-415 in: Biology and Control of Take-All, M. J. C. Asher and P. J. Shipton, eds. Academic Press, New York, N.Y., 538 pp. (1981)). Suppressive soils may be divided into two classes; general antagonism and specific antagonism (Gerlagh, M., Netherlands Journal of Plant Pathology 74:1-97 (1968)).
General antagonism may be found to some degree in all soils and can be directly related to high soil bacteria populations (Rovira, A. D., et al., The nature and mechanism of suppression. Pages 385-415 in: Biology and Control of Take-All, M. J. C. Asher and P. J. Shipton, eds. Academic Press, New York, N.Y., 538 pp. (1981)). Characteristics common to this type of antagonism include the maintenance of soil suppressiveness after heating to 70.degree. C. for 30 minutes, cannot be transferred to other soils, and exhibits greater suppression in undisturbed soils. It is fostered by the addition of organic amendments, increased suppression in soil at temperatures above 25.degree. C., and is promoted by the use of ammonium-nitrogen (NH.sub.4.sup.+ --N) rather than nitrate-nitrogen (NO.sub.3- --N) (Cook, R. J., et al., Biological and cultural tests for control of plant diseases. 3:53 (1988)). Smith (Smith, A. M., Soil Biology and Biochemistry 8:293-298 (1976)) suggested that ethylene (C.sub.2 H.sub.4) biosynthesis by soil microflora may play a major factor involved in general antagonism. Factors supporting the role of ethylene in general antagonism are that ethylene production in soil increases as soil temperatures increase up to 35.degree. C., is promoted by ammonium-nitrogen but inhibited by nitrate-nitrogen, is fostered by the addition of organic amendments, and is greater in undisturbed bulk soils. Ethylene has also been shown to be inhibitory to G. graminis var. tritici at concentrations less than 5 parts-per-million in the soil atmosphere (Rovira, A. D., et al., The nature and mechanism of suppression. Pages 385-415 in: Biology and Control of Take-All, M. J. C. Asher and P. J. Shipton, eds. Academic Press, New York, N.Y., 538 pp. (1981)).
Specific antagonism occurs through continuous monoculture of a crop in the presence of a pathogen (Gerlagh, M., Netherlands Journal of Plant Pathology 74:1-97 (1968)). This results from the buildup of specific antagonistic microbial populations that are antagonistic to the pathogen. This type of antagonism occurs in soils of lower temperature than general antagonism (15.degree.-25.degree. C.), is eliminated by 60.degree. C. moist heat, can be transferred to other soils by mixing, and is related to the build-up of specific bacteria in the rhizosphere (Cook, R. J., et al., Biological and cultural tests for control of plant diseases. 3:53 (1988)).
One of the most studied models of suppressive soils involves take-all disease of wheat and other grasses as caused by the fungus G. graminis. General antagonism to this disease involves all of the factors previously listed. Research interest has been focused on the phenomenon known as "take-all decline" which is a form of specific antagonism in which "suppression (of take-all) develops with 2 or 3 years of wheat monoculture and severe take-all; the soil becomes "immune" to subsequent outbreaks of take-all if cropped exclusively thereafter to wheat and barley" (Cook, R. J., et al., Biological and cultural tests for control of plant diseases. 3:53 (1988)). This occurrence was first reported by Glynne (Glynne, M. D., Annals of Applied Biology 22:225-235 (1935)) in 1935, who noted a reduction in take-all severity after 4 consecutive wheat crops.
Several studies in the mid 1970's correlated fluorescent pseudomonads with the occurrence of take-all decline. Evaluation of 100 bacterial strains for specific antagonism to G. graminis var. tritici in greenhouse conditions by Cook and Rovira (Cook, R. J., et al., Biological and cultural tests for control of plant diseases. 3:53 (1988)), identified eight (8) strains which yielded suppression greater than or equal to those of natural suppressive soils. All eight strains were Pseudomonas spp., seven of which were fluorescent. Further evaluation of bacterial populations by Cook and Rovira (Cook, R. J., et al., Biological and cultural tests for control of plant diseases. 3:53 (1988)) indicated suppressive soils contained 1000 times more fluorescent pseudomonads than non-suppressive soils. Simon and Ridge (Simon, A., et al., Journal of Applied Bacteriology 37:459-460 (1974)) similarly found 100 to 1000 fold increases of fluorescent pseudomonads on infected root tissues than on healthy roots. Agar plate tests demonstrated that over 70% of the fluorescent pseudomonads isolated from suppressive soils were antagonistic to G. graminis. Increases in fluorescent pseudomonad populations have also been linked with the decline of take-all (Gaeumannomyces graminis var. avenae) of turfgrass (Sarniguet, A., et al., Plant and Soil 145:11-15 (1992)). Species of fluorescent pseudomonads that are correlated to the development of soil suppressiveness are P. fluorescens (Weller, D. M., et al., Phytopathology 73:463-469 (1983)) and P. aureofaciens (Cook, R. J., et al., Biological and Cultural tests for control of plant diseases. 3:53 (1988)).
Several mechanisms of pathogen suppression by fluorescent pseudomonads have been proposed. Competition for nutrients and colonization sites has not received recent attention but plays an important role in disease suppression. Bacteria which are capable of using a broad array of nutrients rapidly can reduce carbon and nitrogen sources available for pathogen sporulation and colonization (Weller, D. M., Annual Review of Plant Pathology. 26:379-407 (1988)). Coupled with high metabolism is the ability to undergo rapid reproduction which increases the organisms potential for dispersal and occupation of available niches (Campbell, R., Biological control of microbial plant pathogens Cambridge University Press (1989)). Pseudomonas spp. act in this manner as exemplified by their non-fastidious nature (Palleroni, N.J., Pseudomonadaceae. pages 141-219 in: Bergey's Manual of Systematic Bacteriology, Volume 1, Kreig, N. R. and Holt, J. J., eds. Williams and Wilkins, Baltimore, Md. 1024 pp. (1984)).
The argument for disease suppression by competition emphasizes the importance of colonization in the development of specific antagonistic disease suppression. The degree to which P. fluorescens is able to colonize wheat root tissue can be directly correlated with a reduction in the number of root lesions caused by G. graminis var. tritici (Bull, C. T., et al., Phytopathology 81:954-959 (1991)). Colonization of plant roots by bacteria may be divided into two stages (Howie, W. J., et al., Phytopathology 77:286-292 (1987)). Stage I involves the ability of the bacterium to become attached to the plant root. Stage II is dependent on the bacterium's ability to compete for available nutrients. The ability of a bacterium to colonize root tissue is referred to as its competence. Several traits which may play a role in determining a bacterium's rhizosphere competence include surface polysaccharides, presence of flagella and/or fimbriae, chemotaxis, osmotolerance, and the ability to utilize complex carbohydrates (Weller, D. M., Annual Review of Plant Pathology 26:379-407 (1988)).
Siderophores were the first class of metabolic compounds associated with disease suppression by fluorescent pseudomonads (Kloepper, J. W., et al., Current Microbiology 4:317-320 (1980)). Siderophores are "low molecular weight, high affinity iron (III) chelators" (Weller, D. M., Annual Review of Plant Pathology 26:379-407 (1988)). Under conditions of low iron concentrations, these yellow-green fluorescent compounds are excreted by bacteria and complex with available iron. The bacterium is able to recognize and absorb this complex through membrane receptor proteins. It is believed that siderophores sequester iron thereby making it unaccessible to pathogenic fungi. Support for this mode of antagonism has come from studies in which mutants deficient in siderophore production are less suppressive than the siderophore producing parents (Becker, O., et al., Phytopathology 78:778-784 (1988)). Additional evidence has come from studies indicating that the addition of the synthetic iron chelating compound Fe ethylene-diamine-di-O-hydroxyphenylacetic acid (Fe EDDA) yields disease suppression. Addition of excess iron in the form of ferric-ethylenediamine-tetraacetic acid (FeEDTA) represses siderophore production and eliminates suppressiveness (Weller, D. M., et al., Phytopathology 78:1094-1100 (1988)). Antibiosis by siderophore activity has been linked to antagonism toward Pythium spp. (Becker, O., et al., Phytopathology 78:778-784 (1988)), Fusarium oxysporum (Alad, Y., et al., Phytopathology 75:1053-1059 (1985)), and G. graminis var. tritici (Kloepper, J. W., et al., Current Microbiology 4:317-320 (1980)). Recent work by Hamdan et al., (Hamdan, H., The fluorescent siderophore of Pseudomonas fluorescens: role in suppression of Gaeumannomyces graminis var. tritici and genetic analysis of siderophore production. PhD thesis, Washington State University, Pullman (1988)) involving the generation of siderophore deficient mutants indicate that siderophores have no significant effect on take-all caused by G. graminis var. tritici. Although the role of siderophores in the suppression of G. graminis var. tritici is still in contention, there is little argument regarding it's role in the suppression of Pythium spp. in soil.
Disease suppression of soil pathogens by Pseudomonas spp. has been strongly attributed to the production of antibiotics. Two antibiotics have been attributed to the inhibitory nature of fluorescent pseudomonads, 2,4-diacetylphloroglucinol (DAPG) (Shanahan, P., et al., Applied and Environmental Microbiology 58:353-358 (1992)) and phenazine-1-carboxylic acid (PCA) (Haygood, R. A., et al., Phytopathology 80:435 (1990)). Strain Q2-87 of P. aureofaciens which produces DAPG was identified as being suppressive take-all. DAPG was later confirmed as being a source of antifungal activity of P. aureofaciens Q2-87 on G. graminis var. tritici (Vincent, M. N., et al., Applied and Environmental 57:2928-2934 (1991)).
The production of PCA by P. aureofaciens was first identified by Haynes et al., in 1956 (Haynes, W. C., et al., J. of Bacteriology 72:412-417 (956)). Recent work (Gurusiddaiah, S., et al., Characterization of an antibiotic produced by a strain of Pseudomonas fluorescens inhibitory to Gaeumannomyces graminis var. tritici and Pythium spp. Antimicrobial Agents and Chemotherapy 29:488-495 (1986)) has identified PCA as playing a major role in the inhibitory activity of P. fluorescens against G. graminis var. tritici. Pure crystals of this compound are needle shaped and yellow to green-yellow in color. PCA was shown to be inhibitory to a broad range of fungi with minimum inhibitory concentrations (MIC) to completely prevent fungal growth ranging from 1 to 40 .mu.g/ml on in vitro tests (Gurusiddaiah, S., et al., Characterization of an antibiotic produced by a strain of Pseudomonas fluorescens inhibitory to Gaeumannomyces graminis var. tritici and Pythium spp. Antimicrobial Agents and Chemotherapy 29:488-495 (1986)). Initial reports of the structure of PCA by Gurusiddaiah et al (Gurusiddaiah, S., et al., Characterization of an antibiotic produced by a strain of Pseudomonas fluorescens inhibitory to Gaeumannomyces graminis var. tritici and Pythium spp. Antimicrobial Agents and Chemotherapy 29:488-495 (1986)) proposed that PCA occurred as a dimeric molecule. However, this structure of the antibiotic was later revised by Brisbane et al (Brisbane, P. G., et al., Antimicrobial Agents and Chemotherapy 31:1967-1971 (1987)), who showed that the antibiotic existed in a monomeric state rather than as a dimer.
Thomashow and Weller (Thomashow, L. S., et al., Journal of Bacteriology 170:3499-3508 (1988)) demonstrated the importance of PCA in biological suppression of take-all by P. fluorescens 2-79 through the generation of mutants deficient in PCA production by transposon mutagenesis. All PCA deficient mutants were unable to inhibit G. graminis var. tritici in agar plate tests and provided significantly lower levels of disease suppression in greenhouse studies. Similar studies by Pierson and Thomashow (Pierson, L. S., et al., Phytopathology 78:1522 (1988)) illustrated similar results with P. aureofaciens strain 30-84. Thomashow et al (Thomashow, L. S., et al., Applied and Environmental Microbiology 56:908-912 (1990)) were able to quantify the production of PCA by P. aureofaciens and P. fluorescens on wheat roots grown in steamed and natural soil in the greenhouse. Concentrations of PCA detected in the steamed and natural soils were up to 578 and 133 ng/g root, respectively. PCA was also recovered from the roots of seed treated plants grown in wheat fields (5-12 ng/g root) and in virgin fields (19-27 ng/g root). These results demonstrated the production of antibiotics in the environment and also confirmed that very small amounts of antibiotics delivered to the microsite by biological control agents can be effective in disease management.
The utilization of antibiotics produced by microorganisms such as Penicillium sp (penicillin), Streptomyces griseus (streptomycin), Streptomyces erythraeus (erythromycin), Cephalosporium acremonium (cephalosporin), and several others (Pelczar, M. J., Jr., et al., Microbiology. McGraw-Hill. New York, N.Y. 952 pp. (1977)) for medical uses have been well documented. The potential for application of antibiotics as chemical treatments for plant disease has recently been explored. Application of culture filtrates of Bacillus subtilis provided better management of bean rust than the fungicide mancozeb (Baker, C. J., et al., Plant Disease 69:770-772 (1985)) and protected peaches from infection by Monilinia fructicola (Pusey, P. L., et al., Plant Disease 68:753-756 (1984)). Melvin et al., (Melvin, P. P., et al., Hortscience 28:195-196 (1993)) demonstrated that application of the antibiotic faeriefungin produced by Streptomyces griseus var. autotrophicus provided management of summer patch disease of turfgrass caused by Magnaporthe poae equal to that of the chemical fungicide fenarimol.
Dollar spot is caused by the fungal organism Sclerotinia homoeocarpa F. T. Bennett (Bennett, F. T., Annals of Applied Biology. 24:236-257 (1937)) and is one of the most prevalent diseases of turfgrasses throughout the world (Smiley, R. W., et al., Phytopathology 76:1160-1167 (1985)). It is also the most economically significant disease of turfgrass in the United States and parts of Canada (Goodman, D. M., and Burpee, L. L., Phytopathology 81:1438-1446 (1991); and Vargas, J. M., Management of turfgrass diseases. Burgess Publishing Co. Minneapolis, Minn. 204 (1981)). Inclusion of this organism in the genus of Sclerotinia is currently on a provisional basis. The dollar spot organism produces a flat stroma unlike the sclerotia characteristic of the genus Sclerotinia. Recent work examining the protein composition and anatomy of the stroma (Kohn, L. M., et al., Canadian Journal of Botany. 67:371-393 (1989)) as well as the utilization of electron microscopy and immunological comparisons (Novak, L. A., et al., Applied and Environmental Microbiology 57:525-534 (1991)) suggested that Sclerotinia homoeocarpa should not be included in the genus Sclerotinia. However, proper classification of this organism is not possible as the fertile teliomorph stage of the life cycle is rare or no longer exists. Reexamination of data recorded from fertile teliomorphs collected by Bennett suggested that this organism would be better classified in either the genus Lanzia Sacc. or Moellerodiscus Henn. (Smiley, R. W., Compendium of turfgrass diseases. The American Phytopathological Society, St. Paul, Minn. 102 pp. (1992)).
Turfgrass infected with Sclerotinia homoeocarpa exhibits a bleached or straw-colored appearance. On golf course putting greens and fairways dollar spot lesions appear as sunken spots less than 5 cm in diameter. Under sufficient disease pressure these spots will coalesce and form larger irregular patches. Infected blades initially appear chlorotic and water soaked which become bleached or tan bands traversing the width of the blade with brown margins. Under favorable growth conditions following nights with dew formation, "cobwebs" of white fuzzy mycelium of the fungus may be seen on infected turf (Smiley, R. W., Compendium of turfgrass diseases. The American Phytopathological Society, St. Paul, Minn. 102 pp. (1992); and Vargas, J. M., Management of turfgrass diseases. Burgess Publishing Co. Minneapolis, Minn. 204 (1981)).
High humidity and temperatures ranging from 15.degree. C. to 30.degree. C. are favorable for dollar spot. Dew formation is also conducive to disease development. Much of dew formed is actually guttational water produced by the grass. This guttational water contains carbohydrates and amino acids which may be used as nutrients by the pathogen promoting it to grow and spread. Disease epidemics in Michigan occur in July followed by a second outbreak in late August and early September. The presence of these two separate epidemics may suggest that there may be more than one species of dollar spot, one of which is virulent at temperatures under 20.degree. C., and the other is active at higher temperatures with cool nights (Vargas, J. M., Management of turfgrass diseases. Burgess Publishing Co. Minneapolis, Minn. 204 (1981)). Factors which may increase disease severity include low nitrogen fertility and dry soil conditions (Couch, H. B., et al., Phytopathology 50:761-673 (1960)). Spread of the disease occurs through hyphal growth, and the transport of infected tissue by maintenance equipment and people.
Grasses susceptible to dollar spot in cool weather climates are primarily creeping bentgrass (Agrostis palustris Huds.) and annual bluegrass (Poa annua L.) although Kentucky bluegrass (Poa pratensis L.), perennial ryegrass (Lolium perenne L.) and fescues (Festuca spp.) may also be infected. Susceptible grasses in warm weather climates are bermudagrass (Cynodon dactylon (L.) Pers.), zoysiagrasses (Zoysia spp.), bahaigrass (Paspalum notatum Flugge.), centipedegrass (Eremochloa ophiuroides (Munro.) Hack.), and St. Augustinegrass (Stenotaphrum secondatum (Walt.) Kuntze) (Vargas, J. M., Management of turfgrass diseases. Burgess Publishing Co. Minneapolis, Minn. 204 (1981)).
Cultural practices may be implemented to reduce disease severity. Maintenance of moderate to high nitrogen fertility will reduce disease intensity and promote plant growth and recovery. Irrigation should be used to maintain adequate soil moisture. Irrigation in the early evening should be avoided in order to decrease the duration of leaf wetness. Removal of guttational water by mechanical means or through irrigation promotes rapid drying of the turf and washes guttational nutrients from the leaf surface. Resistant cultivars of susceptible turfgrass species have not yet been identified or developed. Varieties of turfgrass highly susceptible to dollar spot should not be used.
The use of chemical fungicides as preventative and curative treatment of dollar spot are significant disease management tools. Chemicals currently recommended to manage dollar spot are the contact fungicide chlorothalonil and the systemic fungicides propiconazole, fenarimol, iprodione, triadimefon and vinclozolin (Smitley, D., et al., Insect, weed and disease management on commercial turfgrass. Mich. State University, East Lansing, Mich. 28 pp. (1993)).
Despite the effectiveness of chemical fungicides, S. homoeocarpa has developed resistance to several classes of these chemicals. Resistance to fungicides is generally characterized by reduced duration of fungicide effectiveness to a complete failure to manage disease. One of the first studies of resistance to fungicides by S. homoeocarpa was conducted by Cole et al (Cole, H., et al., Phytopathology 58:683-686 (1968)). Their findings demonstrated the presence of strains of dollar spot which exhibited reduced levels of sensitivity to the fungicide thiram and identified strains which were 100 times less sensitive to the fungicide cadmium succinate than sensitive strains. Warren et al (Warren, C. G., et al., Phytopathology 64:1139-1142 (1974)) reported the first case of resistance of S. homoeocarpa to the benzimidazole class of fungicides (benomyl, thiabendazole, and methyl and ethyl thiophanate). These resistant strains exhibited tolerance to these fungicides over 100 times that of the sensitive strains. Due to the widespread development and persistence of resistance to this class of fungicides, they are no longer recommended for the management of dollar spot (Vargas, J. M., Jr., et al., Phytopathology 82:1069 (1992)). The next systemic fungicides developed to manage dollar spot was iprodione, which belongs to the carboxamide class of fungicides. Detweiler et al (Detweiler, A. R., et al., Plant Disease 67:627-630 (1983)) identified a strain of S. homoeocarpa which showed resistance to iprodione at levels 100 times that of the sensitive strains. This iprodione resistant strain also exhibited resistance to the benzimidazole class of fungicides. The most recent class of systemic fungicides released to manage dollar spot are the demethylase inhibitors (DMI) which include the fungicides triadimefon, fenarimol, and propiconazol. Although resistance has been slow to develop, reduced sensitivity has been identified (Vargas, J. M., Jr., et al., Phytopathology 82:1069 (1992)). All fifteen isolates resistant to the DMI fungicides examined have shown resistance to the benzimidazole class of fungicides with two of the isolates showing reduced sensitivity to iprodione. With resistance or reduced sensitivity to all three classes of systemic fungicides used on turf the only effective treatment for these strains remains the use of the contact fungicide chlorothalonil.
One alternative to the chemical fungicides currently being examined is the utilization of organic sources of nitrogen fertilizers for dollar spot disease management. Cook et al (Cook, R. N., et al., Plant Disease Reporter 48:254-255 (1964)) noted differences in the incidence of dollar spot with the use of different nitrogen sources. In these tests an organic form of nitrogen, activated sewage sludge, significantly reduced disease incidence in relation to urea, ammonium sulfate, and ammonium nitrate. The effectiveness of activated sewage sludge in reducing dollar spot incidence was supported by Markland et al (Markland, F. E., et al., Agronomy Journal 61:701-705 (1969)) in a study comparing seven different nitrogen sources. Examination of several compost and organic fertilizers for suppression of dollar spot was conducted by Nelson and Craft (Nelson, E. B., et al., Plant Disease 76:954-958 (1992)). In this study fertilizers were applied as topdressings mixed with 70% sand. Two organic fertilizers (plant and animal meal) tested provided disease management significantly equal to that provided by application of the chemical fungicide propiconazole. Suppression of disease was effective for 30 days after application.
Research into alternative management strategies of dollar spot has also been directed toward the application of biological controls. One of the initial cases of biological control of dollar spot involved application of the fungus Gliocladium virens, which has been used in greenhouse conditions to reduce damping off by Pythium spp. and Rhizoctonia spp. (Haygood, R. A., et al., Phytopathology 80:435 (1990)). Bi-weekly applications in the form of alginate prill pellets of G. virens spores resulted in reductions of disease severity ranging from 46% to 70% (Haygood, R. A., et al., Phytopathology 80:435 (1990)). Goodman and Burpee (Goodman, D. M., and Burpee, L. L., Phytopathology 81:1438-1446 (1991)), reported significant reductions in disease intensity with top-dressing applications of a sand-cornmeal mixture amended with a fungal strain of Fusarium heterosporum, which was isolated from a dollar spot lesion on fine-leaved fescue (Festuca sp.). Significant disease reduction was evident with applications at four week intervals. Topdressing applications at one week intervals resulted in 86% to 98% reductions in disease intensities. The mode of antagonism expressed by this organism is likely to be based on the production of fungi-toxic metabolites. Nelson and Craft (Nelson, E. B., et al., Plant Disease 75:510-514 (1991)), noted significant reductions in dollar spot intensity with topdressing applications of sand-cornmeal amended with strains of the bacterium Enterobacter cloacae. As preventative treatments applied at a 30 day interval, the bacterial topdressing provided a 63% reduction in disease severity which was statistically as effective as the fungicide propiconazole. Application of the bacterial amended topdressing on a curative basis was as effective as the fungicide iprodione in reducing disease severity 12 days after application. The mode of dollar spot suppression by E. cloacae is not understood, but may be related to the ability of the bacterium to parasitize the fungus through adherence to fungal hyphae (Nelson, E. B., et al., Plant Disease 75:510-514 (1991)).